1. Field of the Invention
The present invention relates to an implantable heart stimulator of the type which generates a variable AV interval and which is provided with a basic AV interval.
2. Description of the Prior Art
To reduce the energy consumption of heart stimulators, an automatic threshold search function is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulse and in particular if the sensing is performed by a unipolar electrode configuration.
A fusion beat refers typically in pacing to the ECG waveform which results when an intrinsic depolarization and a pacemaker output pulse occur simultaneously and both contribute to the electrical activation of that heart chamber. Closely related to a fusion beat is a pseudofusion beat that refers to a spontaneous cardiac depolarization occurring at or near a pulse generator output pulse. Because the stimulus occurs after the heart has spontaneously depolarized, the pacemaker output is ineffective, but it distorts the morphology of the complex on the ECG.
Today, fusion beats create a problem for the automatic threshold search function since these beats often are not detected as heart beats. Instead the heart stimulator interprets the evoked response as a loss of capture and as a consequence a backup pulse is issued and the stimulation pulse amplitude is increased. Following undetected fusion beats the heart stimulator might deliver back-up pulses (high output mode), until the next threshold search is performed. This misinterpretation by the heart stimulator of the evoked response signal will, of course, increase the current drain and decrease the lifetime of the battery and the automatic threshold search function will be disabled for some time.
U.S. Pat. No. 5,713,930 discloses a dual chamber pacing system and method with control of the AV interval. The AV interval is adjusted to provide for an optimal AV setting for a selected pacing application. A ventricular fusion test is performed, wherein variations in QT interval are monitored corresponding to variations in the AV interval. Based upon the AV-QT data, the pacemaker can determine the ventricular fusion zone where the pacemaker AV interval is substantially the same as the intrinsic conduction interval, as well as the knee where AV intervals just shorterthan the ventricular fusion zone result in full capture. One application of the dual chamber pacemaker disclosed in U.S. Pat. No. 5,713,930 is for patients with intermittent AV conduction or occasional AV block where it is desired to set the AV delay to be just greater than the natural conduction interval, so that spontaneous beats are permitted.
It should be understood that the present invention is directed to an implantable heart stimulator having, inter alia, an inhibiting function, which means that if intrinsic heart activity is detected, in the atrium or in the ventricle, no stimulation pulse is generated. This means that, using the commonly accepted terminology, the AV-interval could be started by an intrinsic atrial heart activity, a P-wave, and the started interval is then a PV-interval. Thus, instead of writing PV/AV-interval is the term AV-interval used throughout this application.
With reference to FIG. 1 the behavior of a fusion avoidance algorithm in a commercially available dual chamber heart stimulator will be described. A dual chamber heart stimulator comprising a fusion avoidance algorithm and also a threshold search algorithm (discussed below) is disclosed in e.g. “User's manual for AFFINITY™DR, Model 5330 UR, Dual-Chamber Pulse Generator with AUTOCAPTURE™ Pacing System”, by Pacesetter, Ordering No. 2039782, Part. No 9192000-001, issued in 1998, pages 52-54.
FIG. 1 shows an internal electrogram (IEGM) with a normal paced heartbeat seen as the first complex. “A” and “V” designate the stimulation pulses in the atrium and in the ventricle, respectively, and “Cap” stands for capture, i.e. the applied stimulation pulse in the ventricle was successful. “AV” designates the AV-interval. The next stimulation pulse applied in the ventricle did not result in capture and to ensure safe pacing a back-up pulse is applied a predetermined interval after the stimulation pulse. The loss of capture could be the result of an intrinsic contraction at the same time as the stimulation pulse is applied which is detected as a loss of capture. Another alternative is that the stimulation threshold for the heart tissue has increased.
In order to avoid fusion the AV-interval prolonged is with a predetermined time (in the figure designated as Δ), in other words, the heart stimulator prolongs the AV-interval and waits for an intrinsic activity. In this case, as can be seen in FIG. 1, a fusion beat was detected as loss of capture in spite of the fact that it was an intrinsic beat. The AV-interval is prolonged a predetermined number of times, e.g. 3-6 times.
If instead a loss of capture is the result of an increasing stimulation threshold an IEGM illustrating that case is shown in FIG. 2. The loss of capture (LOC) in the second complex is followed be a prolonged AV-interval AV+Δ to in order to avoid a fusion beat as described above. In this case the AV-interval AV+Δ is timed out and a stimulation pulse (V) is applied to the ventricle. The applied stimulation pulse does not result in capture and a back-up pulse is applied which in turn results in capture. In this case the loss of capture was due to an increasing stimulation threshold of the heart tissue.
A threshold search algorithm may be activated by two consecutive loss of capture.
A preferred threshold search algorithm is illustrated in FIG. 2 where the AV-interval shortened to “AV-short” to override any intrinsic heart activity and the ventricular stimulation amplitude is successively stepped up by a predetermined amplitude step of e.g. 0.1-0.3 V and each unsuccessful ventricular stimulation pulse is followed by a back-up pulse. This is performed until the stimulation threshold is detected, i.e. capture is detected from the ventricular stimulation pulse, and the stimulation pulse amplitude is then set to a value that equals the stimulation threshold plus a working margin, e.g. 0.3 V.
The sinus node in the upper part of the atrium is the heart's own “pacemaker”. In a normally functioning heart a depolarization wave is generated by the sinus node and conducted along the conduction system from the atrium down to the ventricle. The conduction system is briefly heart muscle cells specially adapted to depolarize at a certain frequency in the order of 0.9-2 Hz, corresponds to a heart rate of 54-120 beats per minute. If the conduction system from the sinus node in the atrium down to the ventricle does not work normally it is said that the patient has some kind of AV-block.
There are three major groups of AV-block.
A first degree AV-block is present if all atrial depolarization are conducted to the ventricle but the PQ-interval is slightly prolonged (longer than 0.21 s). There are two types of second degree AV-blocks. In second degree AV-block, Mobitz type I (also called Wenchebach block), the PR-interval increases progressively until an impulse is not conducted to the ventricle. Thereafter the cycle is repeated again.
In second degree AV-block, Mobitz type II, the length of the PQ interval is usually stable, while the blocking pattern may be regular or irregular. This means that, if the blocking pattern is regular, every second up to seventh P-wave is blocked.
A third degree AV-block (complete block) is present when there is no conduction between the atria and ventricles.
FIG. 3 shows an IEGM illustrating the behavior of a dual chamber heart stimulator provided with a fusion avoidance algorithm in combination with a stimulation threshold search algorithm on a patient with a second degree AV-block, Mobitz type 11.
The first complex represents a fusion beat followed by a back-up pulse. The AV-interval is then prolonged with A (see the fusion avoidance algorithm described above) in order to determine if the first complex was a fusion beat. In this case no intrinsic activity is detected in the ventricle because the patient has a second degree AV-block, Mobitz type II (in the following called Mobitz II-block) and thus the prolonged AV-interval is timed out and a back-up pulse is generated. Since the criteria for initiating the stimulation threshold search algorithm is two consecutive loss of capture, the search algorithm is thus initiated. The AV-interval is shortened to AV-short in order to override any intrinsic activity (see complex 3). The applied stimulation pulse was successful and capture was detected. The threshold search algorithm is thereby terminated and the AV-interval is then restored to its original programmed or basic value (see complex 4). In the next complex (complex 5) another fusion beat is present and which is detected as a loss of capture. The AV-interval is prolonged again (as in complex 2) and at the same time another Mobitz II-block is present and the sequence described above is repeated again.
One drawback with the above-described phenomena that occurs when fusion beats are present and in particular in patients with a Movitz II-block is that some patients might feel unpleasant when the AV-interval constantly is changing.
Another drawback is that several high-energy back-up pulses are delivered which consumes energy.